A locking fastener or locking fastener assembly is employed to prevent loosening of a threaded fastener element in a fastener joint. There are numerous types of joints in which locking fasteners or fastener assemblies are not only desirable but necessary to prevent a nut from loosening. One such application is in the axle and wheel nut assembly of a motor vehicle or the like.
In a typical axle and wheel nut assembly, the hub is supported on a spindle by axle bearings which permit the hub, and thus a vehicle wheel, to rotate on the spindle. An axle bearing nut is threaded onto the free end of the spindle and holds the axle bearings and bearing races together in a predetermined relationship. The axle bearing nut must be set in precisely the proper position on the spindle to apply end loading on the bearing races sufficient to avoid excessive play in the bearings but insufficient to overload them, the result of either being possible bearing failure or even loss of a wheel.
Numerous types of nuts with positive locking components are well known. One of the oldest and most common of these is the conventional castellated nut and cotter pin assembly. The disadvantages of these assemblies are numerous. They include the necessity of carefully locating a hole through the axle spindle, of using an extra component, of reduced nut strength, of relatively long installation time and of the difficulties encountered in fine tuning the preload on the bearing races.
Newer developments in locking fastener assemblies include those found in the Anderson, Jr. U.S. Pat. No. 3,762,455, the Grube U.S. Pat. No. 4,812,094, the Burdick U.S. Pat. No. 5,533,849, and the Peterkort U.S. Pat. No. 5,597,278, for example. Of these, the Grube and Peterkort patents are assigned to the same assignee as the present invention, as will be noted.
The Peterkort patent discloses a locking fastener assembly consisting of a flanged nut and a retainer washer loosely seated on the nut's flange. The retainer washer includes a radially inwardly extending tab which is designed to slide axially along a slot in a threaded spindle while preventing the washer from rotating relative to the spindle. A releasable locking clip is positioned to lock the nut to the washer. The locking clip is released by engagement of a wrench socket with a hex-head on the nut so that the nut can be threaded to a desired bearing loading position. When the wrench is removed, the clip interlocks the washer and nut to prevent the nut from rotating.
The aforedescribed Peterkort locking fastener assembly is a highly effective device for use in vehicle wheel assemblies. It is simple and relatively inexpensive. However, its design focuses on limiting end play, not maintaining a constant pre-load.
Other known locking fastener designs include prevailing-torque locking fasteners. Locking action is achieved with frictional resistance induced between mating threads. There is positive resistance to assembly, which maintains throughout fastener seating and tightening. A high residual resistance to loosening remains even if fastener pre-load is lost. Disassembly is even difficult. Complete disengagement in service is highly unlikely. Prevailing-torque fasteners are generally all-metal fasteners with modified threads or fasteners with a separate non-metallic element or one fused to the threads. The former have fewer temperature and environmental limitations than the latter, but the latter do not encounter thread galling and other problems characteristic of the former.